The development of computer hard disk drives demands ever increasingly higher track density, lower acoustic noise, and better reliability under shock and vibrational disturbances. The undesirable characteristics of the currently used ball bearing spindles, such as high non-repetitive runout, large acoustic noise, and high resonance frequencies due to bearing defect, impose severe limitation on the drive's capacity and performance.
The use of a non-contact bearing, such as a hydrodynamic bearing, may overcome the aforementioned limitation. The full film lubrication of a fluid bearing displays significantly lower non-repetitive runout and acoustic noise, and its higher damping provides better resistance to external shock and vibration.
The deployment of the hydrodynamic bearing system in a hard disk drive environment requires that the lubricant be securely sealed inside of the bearing structure under all operating and non-operating conditions in order to prevent performance degradation of the bearing and contamination in the drive. At the same time, the bearing system needs to be easily manufacturable in order to satisfy cost requirements.
Prior approaches in the design of lubricant seals for self-contained hydrodynamic bearing units include surface tension or capillary seals and/or traps, ferromagnetic seals, flow recirculation passages, spiral or herringbone pumping grooves and global flow recirculation of lubricant driven by the centrifugal force and pumping grooves resulting from relative rotation of the components comprising the bearing unit.
Capillary taper seals have been shown to be effective when the, bearing unit is at rest. However, when capillary seals were used under dynamic operating conditions, flow passages or pressure ports had to be provided to balance overall seal pressure. These flow passages are difficult and expensive to manufacture, and their effectiveness diminishes when the size of the bearing unit (and thus the passages themselves) becomes smaller.
Ferromagnetic seals have proven to be vulnerable to leakage under thermal expansion conditions. On the other hand, pumping grooves have been shown to result in undesirable ingestion of ambient air during operation. Flow recirculation passages, either for localized lubricant flow, or for global flow throughout the structure of the bearing unit, involve considerable manufacturing difficulty and resultant high prime cost of the hydrodynamic bearing unit.
As the rotational speed of disk drive spindles increases, the centrifugal force exerted on the lubricant inside of the bearing unit increases, thereby overcoming surface tension of traditional outwardly-tapered capillary seals and resulting in lubricant being pumped out of the bearing unit and depleted.
A hitherto unsolved need has remained for an improved hydrodynamic bearing unit and seal which overcomes limitations including high prime cost and leakage/loss of lubricant.